Crosslinked polymer containing carboxyl group have so far been used as thickeners for various aqueous solutions. Such crosslinked polymers containing carboxyl group include, among others, copolymers of an .alpha.,.beta.-unsaturated carboxylic acid, such as acrylic acid, and a polyallyl ether (U.S. Pat. No. 2,923,692), copolymers of an .alpha.,.beta.-unsaturated carboxylic acid and hexaallyltrimethylene trisulfone (U.S. Pat. No. 2,958,679), copolymers of an .alpha.,.beta.-unsaturated carboxylic acid and triallyl phosphate (U.S. Pat. No. 3,426,004) and copolymers of an .alpha.,.beta.-unsaturated carboxylic acid and glycidyl methacrylate or the like (Japanese Kokai Publication Sho-58-84819).
These crosslinked polymers containing carboxyl group are once dissolved in water and then neutralized with an alkali, and the neutralized viscous liquids are used as thickeners, as suspension stabilizers for emulsions or suspensions, and for other purposes. Therefore, it is necessary to first prepare uniform aqueous solutions. It is thus of primary importance to prevent a formation of undissolved powder lumps on the occasion of dissolution in water. Once an undissolved powder lump is formed, a gel-like layer is formed on the lump surface and this decreases the rate of diffusion of water into the lump inside and makes it difficult to obtain a uniform aqueous solution.
Moreover, the higher the viscosity of the neutralized viscous liquid is, the wider the field of application as a thickener becomes and the lesser the amount required is. Further, for the flowing characteristics such as yield value, the higher the yield value is, the wider the field of application as a suspension stabilizer for emulsions and suspensions, hence the commercial value becomes higher.